Feb. 23, 2011 (Orlando) -- The number of people with kidney cancer in the U.S. has risen steadily since 1975 and, since 1991, the greatest increase has been among younger people, researchers report.

From 1975 to 1990, the number of new cases increased on average by 3.6% annually, says study leader Kenneth G. Nepple, MD, a fellow in urologic oncology at Washington University in St. Louis.

From 1991 to 2006, cases rose on average by 2.9% per year, he says.

Cases increased in all age groups from 1975 to 2006, Nepple tells WebMD. But the proportion of patients diagnosed when they were younger than age 65 increased from 45.9% in 1991 to 55.3% in 2006, he says.

Some of the rise comes from increased detection on CT scans, says Christopher G. Wood, MD, professor of urology at the University of Texas M.D. Anderson Cancer Center in Houston.

"A person comes to the emergency department with a [general] complaint such as belly ache and is given a CT scan. A small kidney cancer that might have gone unnoticed is detected," he tells WebMD.

But that can't explain the trend entirely, because the rise in cases began before use of CT scans started skyrocketing in the 1980s, Wood says.

Kidney Cancer on Rise: Why?

"Some of the increase in cases is real. We're not yet sure why the numbers continue to go up, but we think exposures to environmental factors such as smoking and other carcinogens add up and play a causative role," he says. Wood was not involved with the study.

Nepple says he suspects the epidemic of obesity is also helping to fuel the increase.

The researchers used data from the National Cancer Institute's Surveillance, Epidemiology, and End Results cancer registry database to look at renal cancer trends from 1975 to 2006. The database covers about one-fourth of the U.S. population, Nepple says.

Other findings:

In people aged 20 to 39, new kidney cancer cases rose on average from 4.5% annually during 1975-1990 to 5.2% during 1991-2006.

In contrast, among people older than 79, annual new cases dropped on average from 6.7% in 1975 to 1990 to 0.9% in 1991 to 2006.

Overall, there were 7.4 new cases of kidney cancer per 100,000 adults in 1975 vs. 17.6 per 100,000 adults in 2006.

The study was presented at the Genitourinary Cancers Symposium.

These findings were presented at a medical conference. They should be considered preliminary as they have not yet undergone the "peer review" process, in which outside experts scrutinize the data prior to publication in a medical journal.